Hydraulic pumps and motors



Feb. 11, 1964 FlRTH ETAL 3,120,816

HYDRAULIC PUMPS AND MOTORS Filed April 14, 1960 2 Sheets-Sheet 1INVENTQRS Donald Firth Roger Harvey Yorke Hancock BY IQRL LOC K5ATTORNEY Feb. 11, 1964 D. FIRTH ETAL 3,120,816

, HYDRAULIC PUMPS AND MOTORS Filed April 14, 1960 2 Sheets-Sheet 2INVENTORS Donald Firth Roger Harvey Yorke Hancock awn/44121. (A).T:oc K5ATTORNEY United States Patent 3,120,816 HYDRAULIC PUMPS AND MOTORSDonald Firth and Roger H. Y. Hancock, East Kiihride, Glasgow, Scotland,assignors to Council for Scientific and industrial Research, London,England, a body corporate of the United Kingdom Filed Apr. 14, 1960,Ser. No. 22,335 Claims priority, application Great Britain Apr. 16, 19593 Claims. (Cl. 103-162) This invention relates to hydraulic machines ofthe positive displacement type, whether pumps or motors, in which thereaction to the thrust exerted on a piston by the fluid pressure in acylinder is taken through a slipper or shoe on the piston rod working ona tilted or eccentric reaction surface. The machine may be a swash platemachine, in which the reaction surface is constituted by the inclinedsurface of the swash plate, or a radial cylinder machine in which thereaction surface is a relatively fixed cam or eccentric on the axis ofthe machine shaft.

One source of loss of efiioiency in positive displacement pumps andmotors in which the piston rod bears through a slipper against aninclined or eccentric surface is that, especially at low speeds, theslipper tends to tilt, and this materially increases the risk ofmetal-to-metal contact and high friction.

A positive displacement hydraulic motor or pump according to the presentinvention has each piston and slipper assembly designed to operate witha high degree of hydrostatic balance of pressures between the piston andthe slipper bearing surfacei.e. the pressure of a lubricant film at theslipper surface is substantially a direct function of the bearing load.in this way, metal-to-metal friction and the tendency of the slipper totip or tilt are materially reduced or even eliminated.

To this end, the piston and slipper assembly is conveniently designed sothat oil at the working pressure in the cylinder at any given inst-antis available at the Slipper bearing surface which engages the reactionmembereccentric, swash plate, or the likeand which controls thereciprocatory motion of the piston.

Preferably the piston and the slipper, together with any interveningconnecting rod, are drilled through in register to provide a continuousoil duct which includes a metering restriction at or near the endadjacent the working face of the slipper, and the latter hasoil-retaining recesses adjacent its normally leading and trailing edgesfrom which oil can leak. at a controlled rate dependent on theresistance of the metering restriction.

Practical embodiments of the invention will now be particularlydescribed, by Way of example only, with reference to the accompanyingdrawings in which:

FIGURE 1 is an axial cross-section of a swash plate pump or motor, allbut one piston and cylinder being omitted for convenience;

FIGURE 2 is an enlarged sectional view of a piston and cylinder;

FIGURE 3 is a plan view of the slipper in FIGURE 2;

FIGURE 4 is a fragmentary sectional view similar to -FIGURE 2 showing adetailed modification;

FIGURE 5 is a view similar to FIGURE 3 of a modified form of slipper;and

FiGURE 6 is a sectional view on the line VI-VI of FIGURE 5.

The pump illustrated in FIGURE -1 consists of a main frame having frontand back end plates 1, 2 clamped by four pillars (not shown). Each plate1, 2 carries a journal bearing 4, 5 respectively for a short rigid driveshaft 6. Adjacent the bearing 5 in the back end plate 2, the shaft 6 isformed with a locking taper section 7 on which is looked a cyclinderblock 8. This block is drawn upon the taper by a back-nut 9 on theshaft. The

3,120,816 Patented Feb. 11, 1964 cylinder block 8 contains a number ofcylinders 10 whose axes are mutually inclined inwards towards the backend plate 2. A piston 11 in each cylinder is reciprocable under thecontrol of a normally fixed swash plate 12 carried on trunnions (notshown) by which it can be angularly adjusted on an axis normal to theshaft 6. The swash plate 12. has a central conical aperture 14 throughwhich the shaft 6 passes, the dimensions of this aperture beingsufficient to allow for adjustment of the angle of the swash plate tothe shaft 6. i

The working face of the swash plate is recessed at 15a to accommodate anannular bearing pad 15 and an annular slipper plate 16. The latter isfree to rotate under the frictional drag of slippers 17 each of which isengaged with a respective piston 11. For clarity of illustration inFIGURE 1, only one cylinder Iii, piston .11 and slipper 17 is shown. Thebearing pad 15 is locked against rotation by means of a dowel. Thus, thefriction between the piston slippers 17 and the slipper plate 1 6 causesthe latter to tend to follow the slippers 17 around the shaft 6, whilstthe eccentricity of the plate due to the tilt of the swash plate 12 withrespect to the axis of the shaft 6 causes the slipper-s 17 to trace apath over the working surface of the slipper plate which is not ofconstant configuration. Thus wear of the plate is distributed over anarea greater than the annulus which would be traced by a single slipper17 if the plate 16 were stationary.

Each piston 11 consists of a hollow sleeve 54 (FIG. 2) closed at itsouter end to form a crown 55 and which is a snug fit in the cylinder 1%}and has four external oil control grooves 60, 61, 62, 63. Into the outeror working end 56 of the sleeve 54 is screwed a headed stem 64 having anaxial oil feed bore 65 passing therethrough. The outer end of this stemis shouldered at 66 and has a spigot which passes through an oil seal 68in the piston crown 55. An oil seal 57 prevents leakage of oil betweenthe head 69 of the stem and the end 56 of the sleeve. Coaxially with thebore 65, the crown 55 is drilled through to form an oil duct 67 whichopens into a hemi-spherical socket 5B which forms a seating for ahemispherical pivot 76 integral with the slipper 17. The pivot isretained in the socket 5-8 by means of a spring ring 59.

The hemispherical pivot 70 is flattened at 71 to leave a small pocket 72around the end of the oil duct 67 within the seating 58. A radial oilway'73 is drilled through the pivot and leads into a constriction 74 whichcommunicates with a central recess 75 in the face of the slipper 17 (seealso FIG. 3). This recess can thus receive oil from the cylinder 10through the bore 65 in the stem 64, the duct- 67 in the piston crown 55,the pocket 72, the oilway 73 in the hemispherical pivot 70, and theconstriction 74. Thus it will be seen that the pocket 72 must always beproportioned so that the pivot 7tl never closes the duct 67 at a limitposition of its angular deflection.

The sliding surface of the slipper 17 also has an annular groove 76adjacent its periphery, this groove having no communication with thecentral recess 75, but communicating with the atmosphere through fourradial notches 77, 78, 79, 30. The sliding surface of the slipper 17thus effectively consists of five lands-an inner annulus 81 and fourperipheral pant-annuli 82, 83, 84 and 85. These lands are horizontallyshaded in FIG. 3.

The purpose of the above-described arrangement of oil recess and groovesis to combat the tendency of a slipper 17 to tip on the swash plate dueto friction between the spherical pivot 76) and its seating 58. Thistendency increases with increase of thrust between the slipper and theswash plate, which occurs over approximately one half of each revolutionof the particular cylinder concerned, and is coextensive with the oildelivery stroke of the piston. Since all the coacting thrust surfacesare fed with oil at delivery pressure (neglecting any pressure drop dueto flow resistance in the circuit 65 57, 72, 73, 74) the load will betransmitted at each bearing point through a film of oil at a pressurewhich is a direct function of the axial thrust on the piston. Thus,there will be a film of oil between the lands 81 84 and the slipperplate 16, and also between the pivot 70 and its seating 58. The wholepiston and slipper assembly 11, 17 is thus substantially hydrostaticallybalanced.

In practice, there will be a pressure drop in the circuit 65 74 due toleakage between the slipper lands 81 84 and the slipper plate 16, butthis can be kept to a very low value by careful design and accuratemachining of the bearing surfaces. Furthermore if the clearance betweenthe bearing surfaces at any point increases so that the pressure in thecentral recess 75 falls, the hydrostatic balance of the piston andslipper assembly is upset. The pressure on the piston 11 thereupon tendsto reduce the clearance until equilibrium is restored. If the slipper 17tends to tilt about a point on its leading edge, so that the adjacentarc of the slipper tends to make metal-to-metal contact with the slipperplate 16, the moment of the piston thrust about this point opposes thistilting, so that the system is self-compensating to a considerabledegree.

The degree of self-compensation is partly dependent on thecross-sectional area of each notch 77-80 which is presented to theescaping oil, and by careful design of these notches and theirinterconnecting annular groove 76 a dashpot action can be induced inwhich the oil pressure changes are slower than the changes in mechanicalthrust, so that oscillation of the slipper 17 is minimised.

FIGURES and 6 illustrate a modified oil film control arrangement. Here,there are three symmetrically arranged recesses 75a, 75b, 75c fed withoil through metering ducts 74a, 74b, 740. The recesses are of sectorshape, and of equal radius and angle, and are surrounded by a separateconcentric groove 76 which communicates with atmosphere through threeequiangularly spaced notches 77a, 78:2,7911. This arrangement ofrecesses and groove forms four lands, the central land 81a resembling inshape a three-spoked wheel and the outer lands 82a, 83a, 84a beingpart-annular in shape.

The operation of the modified slipper, however is somewhat differentfrom that of the slipper described with reference to FIGURE 3, in thattilting is now opposed by differential hydraulic pressures across theface of the slipper.

For example, let it be assumed that the slipper 17 in FIGS. 5 and 6 ismoving over the slipper plate 16 along the line of the section planemarked VI-VI in FIGURE 5, the notch 79a leading. There will be atendency for the ends of the lands 83a and 84a on either side of thenotch 79a to dig into the surface of the slipper plate 16, while pointson the lands 81a, 82a diametrically opposite the notch 79a will tend tolift. As soon as any such displacement commences, the clearance betweenthe land 81a and the slipper plate 16 will increase and oil will escapeinto the annular groove 76 and out through the notches 77a, 78a. Itspressure at this zone will accordingly be reduced, whilst at thediametrically opposite side of the annular land 81a the clearance willbe reduced and the oil film pressure will be increased. This change inpressure produces a restoring couple so that the modified system ishydrostatically self-compensating.

In both forms of slipper, the surface areas of the lands,

together with their shapes and positions on the slipper surface, arechosen so that the forces between the slipper 17 and the slipper plate16 on the swash plate 12 during a delivery stroke of the associatedpiston 11, which forces tend to promote metal-to-metal contact betweenthe parts, are counterbalanced by forces in the oil films which separatethem, these latter forces being directly derived from the instantaneouspressure of oil in the cylinder 10,

A similar hydrostatic balance is not quite achieved by the hemisphericalpivot and coacting seating 58 as shown in FIGURE 2, since the projectedarea of the latter on a plane normal to the piston axis is less than thearea of the working face 56 of the piston. Consequently, if hydrostaticbalance is to be achieved at this point also of the piston-slipperassembly, the arrangement of FIG- URE 4 is adopted. In this figure, thehemispherical male pivot 7th: is formed on the piston 11 and thecoacting socket seating 58a is formed on the slipper 17, the projectedarea of the seating on a plane normal to the piston axis being madeequal to the area of the working face 56 of the piston. By thisarrangement the surfaces can be proportioned so that a similar balancebetween mechanical and hydrostatic forces is achieved as at the workingface of the slipper 17.

It is to be understood that, with appropriatedesign modifications, thepump described above may be used as a motor.

We claim: I

1. In a positive displacerent hydraulic machine, the combination with areciprocable piston of a slipper articulated to said piston; a pistonthrust reaction member on which said slipper is slidable; at least twoseparate oil retaining recesses symmetrically disposed with respect tothe midpoint of the sliding face of said slipper; each of said recessesextending from a point adjacent the center of said sliding face towardsthe periphery thereof and bounded on all sides by a continuous land;separate ducts communicating between the pressure side of said pistonand said recesses; and a metering constriction in each duct forcontrolling the supply of working fluid to said recess whereby saidslipper is restored to its normal operating position by at least one ofsaid recesses when said slipper is moved from said reaction member.

2. The combination as claimed in claim 1 including an annular groovelocated between said recesses and the periphery of said sliding face,and a plurality of symmetrically disposed notches opening from saidgroove through said periphery.

3. The combination as claimed in claim 1 wherein said piston thrustreaction member is a swash plate and further including a frame, a shaftjournalled in said frame, a cylinder block rigid on said shaft, and saidswash plate in said frame.

References Cited in the file of this patent UNITED STATES PATENTS2,608,159 Born Aug. 26, 1952 2,679,210 Muller May 25, 1954 2,731,308Wilcock Jan. 17, 1956 2,757,612 Shaw 1 Aug. 7, 1956 2,769,393 Cardillo aa1 Nov. 6, 1959/- 2,901,979 Henrichsen Sept. 1, 1959 2,925,046 BudzichFeb. 16, 1960 FOREIGN PATENTS 246,097 Germany June 10, 1909

1. IN A POSITIVE DISPLACEMENT HYDRAULIC MACHINE, THE COMBINATION WITH ARECIPROCABLE PISTON OF A SLIPPER ARTICULATED TO SAID PISTON; A PISTONTRUST REACTION MEMBER ON WHICH SAID SLIPPER IS SLIDABLE; AT LEAST TWOSEPARATE OIL RETAINING RECESSES SYMMETRICALLY DISPOSED WITH RESPECT TOTHE MIDPOINT OF THE SLIDING FACE OF SAID SLIPPER; EACH OF SAID RECESSESEXTENDING FROM A POINT ADJACENT THE CENTER OF SAID SLIDING FACE TOWARDSTHE PERIPHERY THEREOF AND BOUNDED ON ALL SIDES BY A CONTINUOUS LAND;SEPARATE DUCTS COMMUNICATING BETWEEN THE PRESSURE SIDE OF SAID PISTONAND SAID RECESSES; AND A METERING CONSTRICTION IN EACH DUCT FORCONTROLLING THE SUPPLY OF WORKING FLUID TO SAID RECESS WHEREBY SAIDSLIPPER IS RESTORED TO ITS NORMAL OPERATING POSITION BY AT LEAST ONE OFSAID RECESSES WHEN SAID SLIPPER IS MOVED FROM SAID REACTION MEMBER.